3. On August 26, 2022, Abe bought from Bob the latter's Suzuki Jimny with plate number TTH 5307 for Php500,000.00. The arrangement was for Abe to pay Bob Php100,000 upon delivery of the said vehicle on August 26, 2022, to be followed by installments of Php33,333.40 whenever Abe has the means to pay, until the purchase price is fully paid. Whenever Bob demands payment of what was owed, Abe would always say he has no obligation yet because his obligation to pay is conditioned upon his acquisition of money, and incidentally, he is always out of money.

Suppose in problem number 1, the parties have arranged that their rich friends Carl and Dave will finance the sale and pay Bob Php500,000.00 for the car. It was then orally agreed that after Carl and Dave pay Bob, Abe will pay Carl and Dave Php500,000.00 in 24 monthly amortizations and at no interest. It was agreed that Abe will wire his payments to Carl and Dave's joint account every 5th day of the month without need for demand.

Identify the (1) source of Carl and Dave's obligation to pay Bob; and (2) the extent of the liability of Carl and Dave in relation to their obligation to pay Bob. Cite your legal basis.