I believe it is doable the amount not used in the exchange is subject to taxation based on this description:
A 1031 Exchange allows a taxpayer to defer 100% of their capital gain
tax liability. To do this, the exchanger must buy new Replacement
Property equal to or greater than in value to the property sold and
reinvest all of the proceeds from the sale of their old property.
But what happens if a taxpayer desires to purchase property lower in
value or takes a portion of the cash from the closing of the sale and
only invests a portion of their proceeds towards a 1031 Exchange? The
good news is that these transactions still qualify for tax deferral
under Section 1031 of the Tax Code. They simply become “partial” 1031
Exchanges where the taxpayer has a partially tax deferred transaction
rather than deferring all of their taxes.
The portion of the exchange proceeds not reinvested is called “boot”
and is subject to capital gains and depreciation recapture taxes.
taken from this site: