As an instalment, good enough as far as it goes, but not final; for what it is worth.
“I heard Mr. Parnell accept the Bill of 1886 as a measure that would close the differences between the two countries; but since then he stated that he had accepted it as a pro tanto measure. ... It was a parliamentary bet, and he hoped to make future amendments on it.” —Mr. Chamberlain's speech, April 10th, 1893.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894