US Govt Agency asking to pay for app "outside the Apple payment process"

Dear All,

I've just received an email from a user of my paid app who says he is no longer able to use it on his work phone. He works for a US government agency which I'll abbreviate to GOV below. Apparently they have started using Microsoft Intune for device management and as a result they need the following work-around:

This app is not free to download, so it cannot be added to Intune app catalog through this tool. Reach out to the app developer and explain that GOV uses Apple Business Manager (ABM) and MDM for delivering managed apps to devices. Payment through ABM is not supported by GOV, so the app developer will need to provide a version of the app that's free to download but take payment outside of the Apple ID payment process. GOV's Custom App Store can be access by developers by advertising their app to Organization ID 12345678.

I don't know much about MDM, ABM and Intune, and I would more or less consider this a scam except that I am confident that the person who has sent it really does work for this government agency and does use my app on his personal device.

Is there any possibility that what they are asking for is legitimate?

I suspect that the crucial part is: "Payment through ABM is not supported by GOV", i.e. fundamentally ABM/MDM/Intune can support paid apps, but the financial people at GOV AGENCY have chosen not to support that.

Has anyone else experienced anything like this?