Modal institution is not a condition. (par. 1) but when it is violated, the instituted heir is supposed to forfeit the inheritance, to return indeed anything he may have received together with its fruits and interest, if he should disregard this obligation. (Second par.)
In modal institution, the inheritance can be immediately demanded provided security is given.
When the institution with suspensive condition, even if the heir wants to give security , he will not be allowed to do so and will not be allowed to get the property in the meantime. INSTEAD, the property will be placed under ADMINISTRATION. (Art 880)