The problem could be your field type or simply you are not performing the find correctly. Are you doing this with a script? (I assume what you are trying to do is print this sale - correct?)
corrupted file indexes can also be the culprit. If you have exhausted other options, try re-indexing the field. I know of three ways to do this:
1) bring up the field's options in Manage | Database and turn the indexing off. Close/reopen Manage database and turn it back on.
2) Import all your data into a clone of your file
3) recover the file--could fix/change a lot of other things--so don't do this unless at least item 1 doesn't make any change. If the recovered file works and you can't get the re-indexed file to work, replace your damaged file with an undamaged copy.
Field type is number which is automatically generated. Yes, prearing to print the sale.
It might help to post your script.