Currently I am working at a company A (for the past 10 months). Recently I have received a very good offer from a company B and therefore I am planning to join the company B within a couple of weeks. During the negotiation process with the company B, I quoted amount $40K (not a real number) as a signon bonus by saying that I need to payback around $20K (not a real number) to the company A if I leave the company A within 1 year. The company B agreed to provide $40K as a signon bonus. However, recently I have realized that I only need to pay $10K to the comapny A if I terminate my position within 1 year. I am planning to inform this to the company B. However, I am afraid that they might think that either it is silly or I am putting "a show" as a honest person. However, I will feel bad about it if I hide this fact with the company B. How to deal with this? Please help me.
Update:
I am providing some more information here to clarify some of the comments.
Initially the company B agreed to provide a sign-on bonus of $30K. However, I requested $40K by saying that I needed to pay $20K to my previous employer. The company B has agreed to provide $40K as a sign-on bonus. The amount comes directly to me. Later I have realized that by mistake I calculated the repayment as $20K instead of $10K. This is not intentional.