Issuing unfilled receipt to a customer

I am a businessman running a small stationery shop. About four months ago someone I know who works in the public service came to my shop and asked me to issue him with an unfilled receipt. He offered me TZS 5000 for giving him a blank receipt but did not disclose what he was going to do with it and I also did not ask him. I signed one receipt and gave it to him without inserting in it the goods I sold him and their value. In actual fact I sold him nothing. Last week the law enforcement officers came to my shop with the person I gave the unfilled receipt. They asked me to confirm if I gave the person a receipt which I confirmed. They also asked me if I sold him stationeries worth TZS 20 million which I denied. They then arrested and interrogated me for the offence of forgery of the receipt. During interrogation I was shown the receipt that bears my signature but other particulars in it were not inserted by me. Does giving someone an unfilled receipt amounts to forgery if the recipient of the receipt is the one who inserted the false information in it?
MM, Musoma

In the context of section 22(1)(b) of the Penal Code [Cap.16 R.E 2002], a person who does or omits to do an act for the purpose of enabling or aiding another person to commit an offence is deemed to have jointly committed the offence with the actual perpetrator of the offence. If the investigator has evidence proving that you gave the blank receipt to the public servant for the purpose of enabling him to make false retirement of an official imprest or false official claim, you can be jointly prosecuted with that public servant for the offence of forgery based on the principle of aiding even though you didn’t insert the false particulars in the receipt. The law allows drawing inference of the purpose for giving a blank receipt from the conduct of the offenders. It is uncommon to issue a receipt to a person without selling him any service or goods. You should have doubted why a public servant would pay money to get a blank receipt without buying goods or service from you.