Septic system regulations and repairs/upgrades are usually overseen and permitted by your local Board of Health. The Title 5 is the responsibility of the seller -- however, I think in some cases you can build in the cost of the work to your sale price, and the new owner would have to take care of it (no legal expert here, but I am on my BOH). You should find out from the engineer who has provided the estimate what the costs are, and what each units share will be. Also check into the condo by-laws for your association, and see how this is handled in a sale -- it could be that the cost is passed along from owner to owner in the HOA dues, which might increase if the costs are extreme and there is not enough in escrow. I think at this current time you might have a hard time finding a lender willing to negotiate it, or since you have signed a P&S I am assuming for both properties, any kind of personal loan for the costs.