Is there an idiom that means "accepting a bad business deal out of desperation"? If you can't think of something that means exactly that, can you think of an idiom that means "accepting less than you anticipated to get out of a business deal", or even more generally "trying to get any deal done"?
cut [one's] losses
Not exactly "trying to get any deal done" but one option similar in meaning may be 'cut your losses' which would be more in the context of exiting a situation as it stands rather than incur further losses, when you see that someone else has the upper hand (or just that you can't really improve your position).
to avoid losing any more money than you have already lost:
Let's cut our losses and sell the business before prices drop even further.
You could "cut my losses and take this deal" rather than continue to negotiate and end up in a potential worse position.
to be [held] "over a barrel"
If someone has you over a barrel, they have put you in a difficult situation where you have little choice but to do what they want you to do.
You could say in this case e.g. "I accepted this deal although I knew it was bad, as I realised that they had me over a barrel".
Consider the idiom:
back to the wall
Meaning /definition: to be in a difficult situation, to have the odds against one’s self, to have no way of being able to get away, to have no space to escape, to be caught by someone without having an easy escape route, to be trapped in a situation.
I had my back to/against the wall when I was forced to close the deal.
Source: The Idioms
While not specific to business, the phrase "caught between a rock and a hard place" could be used to illustrate making a decision between two undesirable options.
caught between a rock and a hard place
Facing two equally unpleasant, dangerous, or risky alternatives, where the avoidance of one ensures encountering the harm of the other.
Similar to "the lesser of two evils".
the lesser of two evils
The somewhat less unpleasant of two poor choices
"hobson's choice" is one such idiom, meaning to accept a likely unpleasant option or none at all. In full it would be "to make Hobson's choice or "accept Hobson's choice" or similar. Originating (possibly apocryphally) with a stable owner that let the customer take the horse currently nearest the door or none at all.