Q: I asked my mechanic if he could sell my car for $4,000. He ultimately sold it for $4,300. Who gets the extra $300?
A: The mechanic is your agent to sell (sarsur). If the sarsur sold for more than the owner stated, the additional profit is the owner’s. However, had the sarsur initiated the sale and offered to sell on your behalf for whatever price you set, there is a dispute whether he is like any other agent and additional profit is yours, or since he initiated the sale at the price that you set, the implicit stipulation of the sarsur is that the extra is his (Beis Yosef and Bach 186:2).
If you offered the mechanic that anything beyond $4,000 would be his, even if he was silent and didn’t agree explicitly, presumably he agreed to the offer and the $300 is his. You cannot claim that he did not accept your offer. Conversely, he becomes a shomer sachar on the car (C.M. 185:4; Sma 185:5).