I'm not sure if negative cash affects retunue reinforcement, actually, I don't think I've ever been broke while having a substantial retinue. Anyway, the control is on the military tab, go to retinue and there are three round buttons you can click to have it reinforce fully, at half rate, or not at all. I don't usually pay attention to levy reinforcement, but it seems like they mostly reinforce at the castle you raised them from (so you can't get those reinforcements without disbanding and reraising), but they might reinforce a little in the field too, but if so they don't do so fast enough to make much difference from my experience.