pipe sags underneath my townhouse. results in horrible drainage of my downstairs bathroom. home owners association refuses to pay because the problem starts underneath my unit. plumber sais it will cost 13 THOUSAND dollars to repair it. Does anyone know of any other way to repair the problem, other than to digup the floor and re-route the pipe

Go to your county building and find the local estate authority officers. Do you own it? Who did you buy it from? The county would have record of it and you could track that down see what you get.
Or who were the builders? They would be held accountable.

If you don’t own it, the landlord is responsible.

Good luck.