No, Rubio will be not be able to successfully sue and collect the $1,000 later.
Rubio cannot in anyway sue Kamiar for the amount of $1,000 which is the remaining balance based on the fact that Kamiar has offered to pay $4,000 early in full settlement of the debt.
Therefore if Kamiar pays the the amount of $4,000 early as he has offer to pay the amount in full settlement of the debt this means that Kamiar has change his legal position and cannot be sue by Rubio.
Inconclusion no, Rubio will be not be able to successfully sue and collect the $1,000 later.
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