A user is on your ₦1,000/month plan. On day 15 of a 30-day month, they upgrade to ₦2,000/month. They've already paid ₦1,000 for this month and used half of it. Two things must happen: credit them for the ₦500 they didn't consume of the old plan, then charge them prorated ₦1,000 for the rest of the month at the new plan rate. Net: they pay you another ₦500 today. This 'pay the difference' shape is what every SaaS-shaped fintech ships. The math lives in your application layer; the ledger reflects the NET effect on cash and deferred revenue.