Legal Question in Real Estate Law in California

Once the sale is complete, the sale is final unless the borrower/owner files a lawsuit and can show some legal grounds to set the sale aside.

The foreclosure process can be halted and the loan reinstated by paying theprincipal, interest, penalties and costs then due, on or before the fifth business day before the foreclosure sale. Lenders will sometimes accept full payment a day or two later, but are not required to do so and don't count on it. After the sale and delivery of the trustee's deed to the buyer, the sale is substantially final and can only be set aside for certain types of fraud or substantial procedural defects.

Delivery and recording of the trustee's deed is said to "relate back" to the date of sale, so a short delay will not affect the finality of the sale.

Bankruptcy of the borrower will also bring foreclosures to a (sometimes temporary) halt if filed before the sale, but not after the sale.

Just a further clarification to Mr. Whipple's answer. You can't even offer full payment to the lender if the sale has gone through to a third party.

The trustee's sale forecloses the equity of redemption as pointed out by Mr. Whipple. That is actually what the term "foreclosure" actually means.