Require fixed deposit only for payment , rest will be cash

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I'm working on project that I need to have only fixed deposit is paid and that will mark the payment process as done , but I can't know how to make that yet , most of the payment apps doesn't provide that . 

Example for what I need : I have a product X and it's price is 100k , for all products in these collection the customer needs to pay only 2k online , the rest will be cash or another ways .

Any help how to make that custom payment ?