Explain or state briefly the rule or reason for your answer.

1. D owes C P20 000. Both are house painters. It was agreed that D instead of paying C, will paint the house of E to which E expressed his conformity to C whose service as painter was previously contracted by E. Has B the right to enforce the stipulation between C and D?
2. X made an office desk for Y. X told Y that the latter may pay whatever amount he would like t pay for it. When the desk was finished Y offered to pay P2,000 but X instead demanded P3,000, its fair value. Is X entitled to P3,000?
3. S sold to B for P100,000 a parcel of land belonging to S located in his hometown without specifying its exact location and area. Is the sale valid?
4. S has several pigs. Under a contract of sale, S binds himself to deliver a pig to B for P3,000 if the pig has of a the weight of at least 30 kilos. State the binding effect of the sale.

Thank you!