Grammar, Usage / My law assignment?? I dont understand [2]

I)John owes Merry $5000. John supposed to pay Merry in full in 1 june 2006 but on

1 may 2006, John tell Merry to accept a lesser sum of $4000 from John as full and

final settlement of the sum of $5000 owed, tell Merry that is all he has. Merry

feel that she has no alternative but accept the money($4000) and makes a note for

John that he already pay his debt of $5000. But one month later Merry changes her

mind and demand the remaining sum of $1000 that from John.

1)Can Merry got her right to get $1000 ?
2)What if Merry is the one who ask John to pay his debt one month early and John

only pay her $4000. Can Merry ask for another $1000?

I think that for first question Merry can ask the money because she has no choice

to accept but i doubt with my own answer. And for the second I dont think merry

can ask for $1000. Am i right????

II) on 1 april Carla offer to sell jewellery for $10000 to Tom. On 2 april TOm

accepted the offer and send a postal mail to Carla. But because a delay of post

office the mail reach Carla on 12 april. Meanwhile Carla need money already sell

the jewellry on 8 april to Sam.

Can Tom sue Carla ? I have no idea with this case!!!!

Can I find the any principle law in for my question ? ANy info ??