What's your source for this?
They're both only valid via Worksop.
Sheffield to Mansfield
Sheffield is a routeing point, Mansfield is a member of both Nottingham and Worksop routeing points and thus the fares check rule applies. We cannot use Off Peak Returns as the OP is proposing to use as there is not an OPR from Sheffield to Worksop so we will have to use the Anytime Day Return
Sheffield to Mansfield SDR- £10.40
Sheffield to Nottingham SDR- £15.40
Sheffield to Worksop SDR- £5.30
Therefore the only routeing point combination we can use is Sheffield to Worksop.
The mapped routes Sheffield to Worksop is very easy as there is only one map- SH and this only allows you to go direct Sheffield to Worksop and then down the Robin Hood Line to Mansfield.