The phrase "to guarantee that amount" means to promise or ensure that a specific quantity or sum of something will be provided or returned. Full definition
Most policies do guarantee some amount of death benefit, should the investments fall below a specified level. (einsurance.com)
You are also guaranteed an amount at the end of the endowment period. (policybazaar.com)
This doesn't guarantee the amount of pension income they receive, as this will depend on the type of investment the pension scheme is in. (smallbusinessesdoitbetter.com)