You will not settle this by feeling your way through the conversation. Libra's talent is finding the phrasing that keeps everyone comfortable, and that talent will quietly edit your request until it sounds like a favor you are lucky to receive. So do the test on paper first. Four questions, answered honestly, separate a real agreement from another round of accommodation dressed up as compromise.
First: can you say the boundary in one sentence without softening it? Write it flat. "I will not cover weekend shifts anymore." "We split the deposit this month or we postpone." If the sentence only works when you add "if that's okay" or "only if you're up for it," you have written a request for permission, not a boundary. Read it back and notice the urge to shrink it.
Second: what happens inside you when you imagine the other person's face falling? The Venus retrograde square Mars signature is exactly this moment — Venus wants to restore the peace, Mars wants to be taken seriously. If your answer is "I take it back," that is your pattern, and the new moon gives you a clean chance to interrupt it. Not by being harsher, but by pausing before you speak instead of filling the silence for them.
Third: whose need is this, actually? Under a Libra new moon it is easy to draft an agreement that flatters your diplomacy while delivering what the other person wanted from the start. Ask what you get from the terms, in concrete terms: hours, money, sleep, privacy, the ability to say no next time. If you cannot name it, the agreement is appeasement with better manners.
Fourth: would you sign this for two weeks, not forever? Saturn retrograde opposite the lunation rewards specific, testable terms. "Let us try this through October 26" is stronger than a sweeping promise neither of you can keep. It also gives you an exit that is not a fight.
None of these prompts control what the other person does next. They tell you whether the terms you are about to agree to are ones you can actually keep — which is the only part of the negotiation you are writing.