You can have the check or you can have the bike rack. I do not owe you both.
Ending our relationship was in your best interest, whether you can understand or accept that or not. The agreed payment for groceries and gas was imprecise and you know that. If you expect to audit your relationships, you may want to keep better track of expenses in the future. I do not owe you both the check and the bike rack, and I requested the bike rack to be returned to me while you had possession of the check, which is a reasonable request.
You took the bike rack.
If you are going to actively withhold my property from me, I do not owe you the things that I mistakenly still have of yours. I don't expect you to understand why that difference in intention is important, because at this point I honestly think it's beyond you. You have acted with a vindictive volition and you leave me no reasonable recourse.
I'm stopping payment on the check.