Some things are so fundamentally God's, you can't even 'vow' them to Him – and sometimes, you can't buy them back either. Why such strictness?
Leviticus 27 deals with dedicating possessions to God through vows. However, the tithe is presented differently. Because the tithe already belongs to the Lord, it can't be 'vowed' in the same way other possessions can. You can't dedicate something to God that is already His by divine statute.
Protection Against Greed
Furthermore, the text implies that while a tithe could be 'redeemed' (paid for in money), there was a penalty: an additional fifth. This wasn't about making money off of people's generosity. Instead, it was a safeguard against avarice. If people could easily redeem their tithes with cash, they might be tempted to offer less valuable produce or animals. The added 'fifth' was a deterrent, ensuring that the Lord's portion wasn't devalued and that people approached this sacred giving with sincerity, not clever shortcuts.