Renting a Car With a Low Credit Limit, but Prepaid in Part

For a bunch of logistical reasons, it's made a lot more sense for me to rent a car than fly back home. I'm renting a car for two days from Dollar; the total rental cost is $503.75, but I prepaid $256.69 of that about a month ago. I would just use my debit card to pay the whole thing (and eat the $500 holding fee), but Dollar requires a return ticket which I do not have. Alas, I'm stuck using my credit card with a $700 CL to rent this thing out. I'm a student that's primarily been working part-time and for cash, so my bank has ostensibly little incentive to raise my credit limit.

Helpful people of r/travel -- is this feasible? Dollar assesses a $200 holding fee on every car rental, but I don't know if this is on the total price ($503.75) or the cost of the rental that is still outstanding ($247.06). The former, a $703.75 charge, would exceed my credit limit and decline, while the latter would comfortably work out to ~$500. Anybody have any experience in such cases, or advice on what to do? Thank y'all in advance.