That is an excellent idea.
There are multiple requirement which must be met to for a business to legally claim a VAT credit for a purchase. Similarly there are multiple legal responsibilities which must be fulfilled when a business charges VAT. It does not just depend on the sign of a line item or any other statistically associated property in Manager.
With that direction from your accountant you can then enter the transactions in Manager. For cash Sales, receipts now behave identically to sale invoices. Similarly for cash purchases, payments now behave identically to purchase invoices. Significantly increasing Managers consistency and decreasing inadvertent user error.
Your example is very similar to the following Private vehicle use - worked example And the journal entry has the same limitation. I’m not sure why you should have to justify using a journal entry to make a correction in any particular case, as that is the key function of journal entries.
The solution is to explicitly specify sale/purchase status either at the transaction or line item level. What is best depends on the degree of flexibility and functionality envision for journal entries in particular if future linkage to a customer or supplier should later be available at the transaction or line item level See Inconsistency in Tax-reporting - #20 by Patch
The current work around is to use zero valued invoices See Zero-amount invoices not posted to cash-basis general ledger Although now a dummy bank account with zero valued receipts and payment could also be used.
Manager program design options
Manager design currently assigns sale/purchase status based on the transaction type being entered. Sales invoices and related effect VAT sales. Purchase invoices and related transactions effect VAT purchases. A clear and predictable system minimizing user inadvertent error and ideal for the majority or users and their transactions. This maximizes data entry efficiency for the majority of users, but not all users.
Counter trade transactions
Counter trade transactions have a sales and purchase component. When using Managers invoice facility this has required entering the other party as both a customer and supplier, issuing a purchase invoice and a sales invoice, then a payment/receipt with line items to accounts receivable and accounts payable. A cash counter trade transaction is entered by a similar but simpler procedure; Enter the sales component in a receipt, the purchase component as a payment (readily done by opening a second browser window so both payment and receipt can be simultaneously entered).
Manager could offer enhanced support for counter trade transactions for both cash and invoice transaction by having transaction level support for counter trade transactions, this would make explicit the two transactions with greater data entry efficiency than the alternate line item level specification of sale/purchase see GST Worksheet not reflecting correctly - #43 by Patch .
Negative Sales and Negative purchase VAT transactions
Negative sales line items do occur in a sales transaction. As can negative purchase line items in a purchase transaction. Examples are discounts / inducements, returns / refunds, and currency conversion adjustments.
Using invoices, debit notes and credit notes these are readily processed in Manager. All result in a negative adjustment, including returns, refunds, discount line items and the examples give above. This processing approach is legally required See [Withdrawn] How to report EU sales made on or before 31 December 2020 for VAT - GOV.UK
Using Receipts and payment it is now possible to do the same thing for cash sales in a virtually identical manner to how invoices function. In contrast in the past it had been impossible to use cash transactions (receipts or payments) to replicate how Managers correctly processes invoice.
In another universe there could be a society where negative value sales or purchase entries were impossible. If that were the case then, Managers processing of invoices would be a bug.