You may have gotten an Advance. But per-dime covers Expenses, so it is paid after the Spenser’s were incurred.
Would you have had to repay the money if the trip was cut short had not occurred at all?
Yes.
I recall that there were other expenses beyond per diem (lodging and meals); those I had to keep closer accounting, such as taxis. I filed those when I got back for good.
I actually once received another check when I filed taxis expenses that were less than the minimum allowed. I recall that the check was for something like sixteen dollars. I must have received advances for those too. I am surprised that I cannot remember the details.. it just didn't matter that much back then. I didn't try to screw the government and I don't think that they screwed me.
I don't recall getting taxi expenses for TDY here it was overseas only. But TDY here I just didn't worry about it beyond the basic lodgings and meals.