You can say no — you just bury it under three paragraphs of justification nobody asked for.
You can say no — that's not the problem. The problem is you can't say it in under three paragraphs. Someone asks for the money again, the third time this month, and instead of "I can't," you deliver a TED talk about your budget, your last paycheck, your therapist's opinion, and your deep respect for them as a person. The over-explaining isn't communication — it's a permission slip you're begging them to sign. Deep down you believe a no only counts if they agree it's reasonable, so you hand them the negotiation. And a boundary you have to justify is a boundary you've already put up for debate.
Get your read — free on iPhoneyour no is real. it's just hiding under a justification so long you've accidentally reopened the negotiation.