We act as a bridge for international investors looking to diversify their portfolios in Europe.If direct investments in Spain become less viable due to these tax implications, an alternative could be collaborating with local companies like ours.

Alternatively, you can tell them you will charge their security deposit for it, but that becomes a charge against your security for the property.You should consider whether you really want them out of the property with regards to the above paragraph.

An alternative could be including photos of plants in your design.

Alternatively, I could explore hard money lenders.

As you're looking at properties it's best to identify multiple potential replacement properties to give you more opportunities and alternatives. 

An alternative is something called Binder. 

Alternatively, if you need liquidity for your San Diego move, explore refinancing options on your current properties to tap into some of that equity.

Your goal is to build a passive real estate portfolio (we can discuss what passive means to you), so while I don't condone throwing $100k into a deal without even a basic understanding, alternatively doing so will teach you far more lessons than reading a book will.  

I'd suggest that there is essentially no tax implication for this as compared to your alternatives.

Alternatively, restructure the loan with equity participation for potential capital gains treatment at lower rates.