Walras's Law V.S Say's Law- Is there a difference?
The way you have defined excess demand, it is only consumer excess demand. But Walras's law holds in any private ownership economy at all prices (at which demand and supply are well defined). Walras's law is basically equivalent to consumers spending their budget fully.

Let there be $l$ commodities, so every commodity bundle is an element of $\mathbb{R}^l$. There are $m$ consumers and $n$ firms. The set of production plans firm $j$ can produce in terms of net-output is $Y_j$. With this convention, if $p\in\mathbb{R}^l$ is the price system and $y\in Y_j$ a production plan, $p\cdot y=\sum_{k=1}^l p_k y_k$ is the resulting profit. Every consumer has an endowment $\omega_i\in\mathbb{R}^l$ and a (potentially zero) share $\theta_{ij}$ in firm $j$ and the resulting profit. Every firm is privately owned, so each firm's shares sums to one, $\sum_{i=1}^m\theta_{ij}=1$.

At the price system $p$, the budget of each consumer is $p\cdot \omega_i +\sum_{j=1}^n \theta_{ij} p\cdot y_j$. If the consumer $i$ demands $x_i$ and spends their whole budget, we have $p\cdot x_i=p\cdot \omega_i +\sum_{j=1}^n \theta_{ij} p\cdot y_j$. Summing up and using that prices are linear,
$$p\cdot\sum_{i=1}^m x_i=p\cdot\sum_{i=1}^m\omega_i+p\cdot \sum_{i=1}^m\sum_{j=1}^n \theta_{ij}y_j$$
$$=p\cdot\sum_{i=1}^m x_i=p\cdot\sum_{i=1}^m\omega_i+p\cdot \sum_{j=1}^n\sum_{i=1}^m \theta_{ij}y_j$$
$$=p\cdot\sum_{i=1}^m x_i=p\cdot\sum_{i=1}^m\omega_i+p\cdot \sum_{j=1}^ny_j$$
$$=p\cdot\bigg(\sum_{i=1}^m x_i-\sum_{i=1}^m\omega_i-\sum_{j=1}^ny_j\bigg)=0,$$
so the value of aggregate excess demand is zero and Walras's law holds. But nothing in the argument requires prices to clear markets so that supply equals demand, 
$$\sum_{i=1}^m x_i-\sum_{i=1}^m\omega_i-\sum_{j=1}^ny_j=0.$$
The latter is clearly a sufficient condition for excess demand to have the value zero, but very far from necessary.

Walras's law is a provable result under some standard assumptions (that guarantee that consumers spend their whole budget), but Say's law is not a formal result in any way.