Will the security instrument be nullified or deemed unenforceable?

The only experience I have in real estate comes from the experience of purchasing my first home in Richardson with my husband, who is a software engineer at Texas Instruments. 

A party of interest in the property that was sold at tax sale may assign his or her right to claim excess proceeds only be a dated, written instrument that explicitly states that the right to claim to claim excess proceeds is being assigned, and only after each party to the proposed assignment has disclosed to each other party all facts relating to the value of the right that is being assigned.

If you purchase with cash, then you can finance the property at anytime.Another "tip" I would suggest is to finance with variable rate (though perhaps fixed term) instruments, but arrange your payments as though you had opted for a fixed-rate financing.  

However I would be worried about liability if I said I'd buy their house for $70K they say no and I say well I could list it for $120K and see what happens then have it on the market a few months and drop the price a coupe times to like $100K then have them getting desperate where they would take the $70K to get out from under it, but now I have a fiduciary duty as their agent.

They agree to those terms and we setup a few legal instruments to protect all parties:1.

Once the docs are drawn up, you get them signed and notarized and have either the attorney or title company record the financing instruments with the county recorder.

It used to be that bonds were an important part of a typical portfolio--until bonds started returning near zero (which was pretty instrumental in pushing stock prices up  as more dollars crowded into the stock market). 

This Agreement and Tenant’s right under it shall be subject and subordinate to the lien, operation and effect of each existing or future mortgage, deed of trust, ground lease and/or any other similar instrument of encumbrance covering any or all of the Premises, if any, and each renewal, modification, consolidation, replacement or extension thereof.19.

In writing off interest I will re-coup about 25% of the interest from my tax return but seems like the other 75% of interest needs to be accounted for.