$12 Tip?

So today I was at work and I had a strong craving for some eggrolls. Now I had been wanting some for the last couple of days and I decided to Google some places around my job to get some. Now the only place I could find was a few minutes walk away and they had 6 mini shrimp egg rolls for $6.50. Now I get to this establishment and there was a takeout window, where they were not even letting you inside and and you had to stand outside in the freezing cold and pick up your order. Then I get the receipt and it was $7 some change. I use my card and there was a 15% tip box that I checked and all together it was $8 some change. Now I get back to my job and is very much disappointed in the egg rolls I see. Now I know it was mini egg rolls but this was honestly so small about the size of my pinky finger and needless to say was gone in a couples seconds. Anyway I was telling one of my customers about the experience and just chatting it up. Then at the end of the transaction he gives me $12 and is like this is for your lunch so you can get something different tomorrow. Now here where the problem comes in, I am at a bank and under no circumstances are we allowed to take cash gifts from customers (especially right in view of camera). So I try to tell him I can’t take it and he of course shakes his head and walks away. Now usually when these situations occurs (it has happened to me a few times working at the bank)I just re deposit it back into the clients account. He did not have an account and so I basically ended up having to give it to the bank in essence. Therefore this brought on a bittersweet feeling, but alas working at the bank if I don’t have my integrity, what else do I have? Moral of the story if you want to tip the bankers give them a gift card. Or buy the actual food😪😪