Below are some case situations on EXTINGUISHMENT OF OBLIGATIONS

Below are some case situations on EXTINGUISHMENT OF OBLIGATIONS. Can you answer them? (with explanation and if possible provide the related law article)

1. X and Y were co-owners of a piece of property worth P1O million. For some repairs thereon, Y paid P2,000,000. Because they were co-owners, X had to share in said expenses, and so X owed Y P1,000,000. X sold his share in the property to Z and Y also sold his share in the property to Z. later, Y brought an action to recover, P1,000,000 from X. X claimed that since Z is now the owner of the property, Z owes himself, and therefore said merger had extinguished his debt to Y. should X pay Y?
2. X owes Y P50,000. Z is the guarantor of X. Y owes X P10,000. When Y sues X and X cannot pay, for how much will Z be liable?
3. X owes Y P50,000. Z is the guarantor of X. Y owes Z P50,000. When Y sues X for the P50,000, may X successfully put up the defense of compensation in that, after all, his creditor (Y) owes Z the same amount?
4. X owes Y P100,000. Y owes X P20,000. Both debts are already due. Later Y, with the knowledge but without the consent (or against the will) of X, assigned the P100,000 credit to Z. how much can Z successfully collect from X?
5. X owes Y P10 million due on April 2; Y owes X P2,000,000 due also on May 4. On March 8 (when there was no legal compensation yet), Y assigned his P10 million credit to Z, with the knowledge but without the consent of X. on april 2, how much Z can successfully collect from X?