"How Do I Tackle This At My Workplace?"

Pls i need a candid advise on how to deal with this situation at my work place.
I got this job August Last year after waiting for years after my NYSC. There is something going on here which is strictly against my believe and practice.
Some senior staff will come with receipts for me to sign. Receipts on goods and services that has not even taken place so that they can forward it to the headquarter for approval. How do i tell them its against my believe cuz i always feel that guilt after signing .
I need advise on how to go about it without offending anyone and without affecting my job.