Is the mortgagee’s letter informing the mortgagor of his intent to foreclose is already considered a foreclosure of the chattel?

No. A mere offer by the mortgagor to surrender the chattel, not accepted by the mortgagee, does not preclude the mortgagee from bringing suit to recover the balance of the purchase price. (Industrial Finance Corp v. Castor Tobias, G.R. No. L-41555, July 27 1977)