An agency must set a single engagement rate for the employee, expressed as a percentage of the employee`s base salary, which must not exceed 25%. The OPM authorisation may increase this ceiling to 50 % on the basis of a critical need on the part of the administration. (See 5 CFR 575.309 (e)) The incentive may be paid in instalments after the end of certain periods of service during the entire period of service prescribed by the service contract or by a single lump sum after the conclusion of the entire duration of service required in the service contract. An agency shall not pay a retention incentive as the first lump sum payment at the beginning of a period of service or before the performance of the period of service for which the retention incentive is received. .