I applied for a software development job in another city in Germany. When I was invited to an on-site interview they also stated that I would get reimbursed for my travel expenses. I paid for a second class train ticket and a few extra bucks for a ticket inside the city (to get from the main station to the company office and back). When I was there I handed in the form for reimbursement and the receipt for both tickets. After a few interviews with different people at the company I decided that I did not longer want to work for them and I messaged them saying that I was withdrawing my application. This was prior to any feedback from them. They reacted quite professionally and they didn't seem unfriendly in their reply.
However, two months later I still did not receive any reimbursement. I would like to message them and remind them about our agreement that I would get reimbursed for my expenses.
I believe I have a right to reimbursement according to §670 BGB (German Civil Code) and they stated that I would get reimbursed anyways (I have this in my inbox still).
How can I put the wording in my email so that it reads like a friendly reminder and nothing more? I would also like to ask them when I would get the money but I'm worried this might not be received well.