You pay it.
Advise the new buyer this is why the monthly dues are so low.
Unless, this was a one time assessment and will not recur.
In that case, pay it and say nothing.
If the buyer was fully aware of the pending assessment, the buyer may have budgeted for it. Now would be a good time to demonstrate your skills of negotiation.
One condo community here in Dunedin, FL, had a $60,000 per unit assessment a few years ago. 30% of the units had to be sold because the owners could not pay the assessment. There are always workarounds or options that aren't linear.
To answer your question I would ask, "If the buyer refuses to pay the assessment and backs out of the deal, who will be responsible for paying the assessment?"