Could you please keep (or hold) THESE jeans at/by the CASH register for me? Jeans, pants, trousers, shorts, etc. are plural. By means some place next to the cash register. At means the cash register.

It should be could you keep *these* jeans.

Both sentences are grammatically correct, but there is a small difference in usage. "Could you keep this jeans by the cashier register for me?" "Could you keep this jeans at the cashier register for me?" The first sentence is a bit more informal and might be commonly used in casual conversation. However, it's important to note that "jeans" is plural, so it should be "these jeans" instead of "this jeans." The corrected sentence would be: "Could you keep these jeans by the cashier register for me?" The second sentence is also correct and is slightly more formal. It uses "at the cashier register," which is also grammatically sound. Again, for proper grammar, it should be "these jeans" instead of "this jeans." So, the corrected versions would be: "Could you keep these jeans by the cashier register for me?" "Could you keep these jeans at the cashier register for me?"

THESE jeans! The problem is those sentences is not the at/by... but the use of THIS.