Timeslips provides billing systems for firms, such as accountants and lawyers, which charge clients by time.
The company is based in Boston, where research and development will remain, but administration will move to Dallas, where Sage has its largest US operation.
The dollars 5.1m price may double if Timeslips reaches two-year sales targets - the deferred element is a maximum of dollars 7m.
In the year to 31 December, Timeslips broke even on sales of dollars 5.4m. However, a spokesman for Sage said that the current ownership structure meant shareholders took most of the profits out of the business.
Timeslips estimates that sales for the current year may be more than dollars 7m and expectations for operating profits this year, before relocation costs, are dollars 1m. The company said Timeslips' software products could be exported from the US to the UK and other European countries whereas much of Sage's existing software, particularly that written for accountants, is country-specific.
Sage bought Multisoft Financial Systems, a software writer, for pounds 4m last month. Its shares were unchanged at 584p yesterday.Reuse content