Magento Wholesaler website - Assign credit money and period to customers

My client is wholesaler and have Magento website. We need functionality like this -

Users (Customers) will have credits (assigned by web-owner), using those credits they can buy goods from site and do not need to make payment. Suppose I am a customer and have credit of £1000 and I want to buy goods worth £800 then I can do so without making any payment.

But the issue arise now - It's not like a customer does not make payment at all using credits. Customers are supposed to make payment after certain period of time like 30 or 45 days. So If I have bought goods worth £800 then I need to make offline payment (by Cheque or bank transfer) within certain period of time. Once web owner receives the payment they go to web admin and make a note to customer's account and it will get customer all credits (£1000) back.

Client also needs to have monthly statement sent each month to their customer.

Please help me getting through this.

About the credits option, do not know if there is any solution out there, but you can implement payment plans by using layaway module in magento. May be you can get credits facility added in the layaway module. checkout the demo of this product if it can work.