Miscellaneous Cash accounting line as a source for Tax accounting string in SLA?

Has anyone else ran into the issue of not being able to use the Miscellaneous Cash accounting line as a source for Tax accounting lines? We have not found a way to automatically populate the tax accounting line from the Misc. Cash accounting line in Miscellaneous receipts because Miscellaneous Cash is not a source that we can use in SLA. We can create a tax line that has the same COA, but it creates the line with the same amount as the Miscellaneous Cash line, which does not accurately capture the amount.