The story says the maternity leave will be an "advance" on future earned leave. I do not know if there is precedent for this. I suspect not. I am aware that unused federal leave can be donated. This is fairly common. Federal employees get fairly lavish leave as it is. People are busy, they find it hard to get away, and leave stacks up. Agencies turn into ghost towns in December as employees in use or lose situations take off, and a great deal of leave is surrendered anyhow. In such situations, donating leave to a coworker with a major medical issue is eminently rational, as well as decent. But I have never heard of borrowing ahead. This would seem to me to involve an expenditure of funds for an as-yet unearned, future benefit, and I would think that would require statutory authority.
Only Annual Leave can be donated.