First, it's possible the message is wrong. I seem to recall getting over salary budget somehow a long time ago. I thought I traded my way out. Can't say for sure, and they've made some changes to the game since then. You'll know in a few hours.
As long as the other GM agrees, you can clean up your salary issue within 2 days and resubmit the trade.
IMO, asking another GM who is supposed to be competing against you to help you out by claiming one of your crap players off waivers so you can free up some salary is cheating / collusion. I'd lobby hard for HBD to toss both of you out of any world I was in since collusion is against the Fair Play rules.
You got yourself into the mess. Get yourself out.
I'm going to question that you "don't" have any money to transfer. I'm guessing you don't want to transfer it. Only way you can't transfer money into salary is if your Prospect budget is $6M before the draft, or below $2M after the draft. Otherwise, you can move money into Salary.
If this trade in important enough, waive one or more players good enough for another team to want them. As long as they are making $500K or more, you can replace them with someone who will make $327K and that will free up $150K.