By binomial approximation, we have that $(1+x)^a=1+xa$ for large values of $a$, since $|ax|<<1$. Now suppose I have the following expression $$-B(1-\frac{x_1}{B})^{-1}$$ for some large value of $B$, we have that $|-\frac{x_1}{B}*-1|<<1$, hence may $(1-\frac{x_1}{B})^{-1}$ be approximated by $1+\frac{x_1}{B}$, such that the first expression is
$$-B(1-\frac{x_1}{B})^{-1}=-B(1+\frac{x_1}{B})$$
provided $B$ is sufficiently large?
$x_1$ is a small positive number (relative to $B$)