One reason for SDGAS is those situations where donors can't be identified, eg a street collection ("bucket/tin collection") where each donor puts their donation and it would be impractical to get their name and address.
The collectors are not necessarily the counters, so the counters will have no idea of the number of donors and the exact amount each donor put in (but then neither will the collectors if the donor puts in a handful of coins). However, if notes are folded together or in an envelope, etc joining them together as a "bundle" then only if the total of that bundle is over £30 can that donation not be claimed towards SDGAS.
The result is that after these bundles have been removed, the total of the collection remaining is all eligible for SDGAS.
It is impossible to enter this aggregated anonymous total (say £200 in a tin) directly into Sage as the SDGAS limit of £30 (or whatever configured) kicks in - except by fiddling the limit to allow the entry and then resetting it back to what it was before.
Having an override option for such "bucket" totals would improve the efficiency of entry.