If a donor transfers by gift less than his entire interest in property, the gift tax is applicable to the interest transferred. The tax is applicable, for example, to the transfer of an undivided half interest in property, or to the transfer of a life estate when the grantor retains the remainder interest, or vice versa. However, if the donor's retained interest is not susceptible of measurement on the basis of generally accepted valuation principles, the gift tax is applicable to the entire value of the property subject to the gift. 

Giving the son cash to buy the house is not an "incomplete gift."  If the son gives the parents a note, promising to pay it back on a certain date, then there is at least imputed interest unless the amount stated is at a rate acceptable to IRS.  The imputed interest may be subject to gift tax, or income tax, or both.