Genesis 44:2

Joseph was giving his brothers the chance to abandon Benjamin if they wanted to. Joseph was testing them to see if they were loyal to the family and faithful to their father.

The brothers seemed different. They felt sorry for what they did to Joseph. They showed honor by returning the money and by bringing Benjamin. Given a chance to get rid of Rachel’s other son, Benjamin, would they do it?

put my cup, the silver cup, in the sack's mouth--It was a large goblet, as the original denotes, highly valued by its owner, on account of its costly material or its elegant finish and which had probably graced his table at the sumptuous entertainment of the previous day.

Joseph heaps further kindnesses upon his brethren, fills their sacks, returns their money, and sends them away full of gladness; but he also exercises them with further trials. Our God thus humbles those whom he loves and loads with benefits. Joseph ordered his steward to put a fine silver cup which he had (and which, it is likely, was used at his table when they dined with him) into Benjamin's sack's mouth, that it might seem as if he had stolen it from the table, and put it here himself, after his corn was delivered to him. If Benjamin had stolen it, it had been the basest piece of dishonesty and ingratitude that could be and if Joseph, by ordering it to be there, had designed really to take advantage against him, it had been in him most horrid cruelty and oppression; but it proved, in the issue, that there was no harm done, nor any designed, on either side. Observe,
I. How the pretended criminals were pursued and arrested, on suspicion of having stolen a silver cup. The steward charged them with ingratitude - rewarding evil for good; and with folly, in taking away a cup of daily use, and which therefore would soon be missed, and diligent search made for it; for so it may be read: Is not this it in which my lord drinketh (as having a particular fondness for it), and for which he would search thoroughly? Gen 44:5. Or, "By which, leaving it carelessly at your table, he would make trial whether you were honest men or no."
II. How they pleaded for themselves. They solemnly protested their innocence, and detestation of so base a thing (Gen 44:7), urged it as an instance of their honesty that they had brought their money back (Gen 44:8), and offered to submit to the severest punishment if they should be found guilty, Gen 44:9, Gen 44:10.
III. How the theft was fastened upon Benjamin. In his sack the cup was found to whom Joseph had been particularly kind. Benjamin, no doubt, was ready to deny, upon oath, the taking of the cup, and we may suppose him as little liable to suspicion as any of them; but it is in vain to confront such notorious evidence: the cup is found in his custody; they dare not arraign Joseph's justice, nor so much as suggest that perhaps he that had put their money in their sacks' mouths had put the cup there; but they throw themselves upon Joseph's mercy. And,
IV. Here is their humble submission, Gen 44:16. 1. They acknowledge the righteousness of God: God hath found out the iniquity of thy servants, perhaps referring to the injury they had formerly done to Joseph, for which they thought God was now reckoning with them. Note, Even in those afflictions wherein we apprehend ourselves wronged by men yet we must own that God is righteous, and finds out our iniquity. 2. They surrender themselves prisoners to Joseph: We are my lord's servants. Now Joseph's dreams were accomplished to the utmost. Their bowing so often, and doing homage, might be looked upon but as a compliment, and no more than what other strangers did; but the construction they themselves, in their pride, had put upon his dreams was, Shalt though have dominion over us? (Gen 37:8), and in this sense it is now at length fulfilled,; they own themselves his vassals. Since they did invidiously so understand it, so it shall be fulfilled in them.
V. Joseph, with an air of justice, gives sentence that Benjamin only should be kept in bondage, and the rest should be dismissed; for why should any suffer but the guilty? Perhaps Joseph intended hereby to try Benjamin's temper, whether he could bear such a hardship as this with the calmness and composure of mind that became a wise and good man: in short, whether he was indeed his own brother, in spirit as well as blood; for Joseph himself had been falsely accused, and had suffered hard things in consequence, and yet kept possession of his own soul. However, it is plain he intended hereby to try the affection of his brethren to Benjamin and to their father. If they had gone away contentedly, and left Benjamin in bonds, no doubt Joseph would soon have released and promoted him, and sent notice to Jacob, and would have left the rest of his brethren justly to suffer for their hard-heartedness; but they proved to be better to Benjamin than he feared. Note, We cannot judge what men are by what they have been formerly, nor what they will do by what they have done: age and experience may make men wiser and better. Those that had sold Joseph would not now abandon Benjamin. The worst may mend in time.

And put my cup, the silver cup. It may seem wonderful that, considering his great opulence, Joseph had not rather drunk out of a golden cup. Doubtless, either the moderation of that age was still greater than has since prevailed, and the splendor of it less sumptuous; or else this conduct must be attributed to the moderation of the man, who, in the midst of universal license, yet was contented with a plain and decent, rather than with a magnificent style of living. Unless, perhaps, on account of the excellence of the workmanship, the silver was more valuable than gold: as it is manifest from secular history, that the workmanship has often been more expensive than the material itself. It is, however, probable, that Joseph was sparing in domestic splendor, for the sake of avoiding envy. For unless he had been prudently on his guard, a contention would have arisen between him and the courtiers, resulting from a spirit of emulation. Moreover, he commands the cup to be enclosed in Benjamin's sack, in order that he might claim him as his own, when convicted of the theft, and might send the rest away: however, he accuses all alike, as if he knew not who among them had committed the crime. And first, he reproves their ingratitude, because, when they had been so kindly received, they made the worst possible return; next, he contends that the crime was inexpiable, because they had stolen what was most valuable to him; namely, the cup in which he was accustomed both to drink and to divine. And he does this through his steward, whom he had not trained to acts of tyranny and violence. Whence I infer, that the steward was not altogether ignorant of his master's design.

Put my cup in the sack's mouth of the youngest - The stratagem of the cup seems to have been designed to bring Joseph's brethren into the highest state of perplexity and distress, that their deliverance by the discovery that Joseph was their brother might have its highest effect.

And put my cup, the silver cup, in the sack's mouth of the youngest,.... Benjamin; this he ordered to be done, partly to put him in apparent danger, and try how his brethren would behave towards him in such circumstances, and thereby know how they stood affected to him; and partly that he might have an excuse for retaining him with him. This cup was valuable both for the matter of it, being of silver, and for the use of it, being what Joseph himself drank out of: and by the word used to express it, it seems to have been a large embossed cup, a kind of goblet, for it has the signification of a little hill. Jarchi says it was a long cup, which they called "mederno". The Septuagint render it by "condy", which is said to be a Persian word, and a kind of an Attalic cup, that held ten cotylae (g), or four or five quarts, and weighed ninety ounces; but a cup so large seems to be too large to drink out of:

and his corn money; what he had paid for his corn:

and he did according to the word that Joseph had spoken; put every man's money in the mouth of his sack, and his silver cup with the corn money into Benjamin's sack.

(g) Nicomachus de festis Aegypt. apud Athenaeum, l. 11. c. 7.

It seems to have been a large cup, and of great price, and much used by Joseph.

In the sack’ s mouth of the youngest, with design to discover their intentions and affections towards Benjamin, whether they did envy him, and would desert him in his danger, as they did Joseph; or would cleave to him; that hence he might take his measures how to deal with him and them.