Loyalty Payment to Customer: time based

Hi, Hoping someone could help with an amortization issue and also any technical reasons why under IFRS and US GAAP. We pay our customers an upfront sign-on bonus (eg: $600), and if they remain our customer for at least 2 years they retain the full $600. If the customer terminates their agreement with us before 2 years they are required to pay back a portion of the bonus, but it is as per a schedule: 1 – 12 months: pay back all the bonus 13 – 24 months: customer ‘earns’ $50 per month, and would pay back bonus as follows if they terminate early: 13: repay $550 14: repay: $500 15: repay $450 ... 23: repay $50 24: no repayment. We are accounting for the bonus as a reduction from revenue amortizing over the 24 months. Initial entry is on payment to the customer is: DR Deferred Expense $600 CR Cash $600 But, should we amortize the DR to revenue: - straight-line over 24 months (ie: $25 per month over 24 months), or - $0 for the first 12 months as the customer has not qualified to retain any of the bonus yet, and will start to ‘earn’ in year 2 so $50 per month for 12 months of year 2 - or some other allocation basis? The bonus is not dependant on any thresholds that the customer spends with us, only that they continue to be a signed-up customer. We account under IFRS but refer to US GAAP where appropriate so any reference to either would be a big help. Thank you,